It is desirable to provide intelligible markings that are virtually invisible to the human eye on the surface of articles for identification, authentication, sorting, etc. U.S. Pat. Nos. 5,093,147; 5,336,714 disclose using certain near infrared fluorescent compounds having minimal light absorption of radiation in the visible range 400-700 nanometers (nm) and strong light absorption in the near infrared region about 700-900 nm with accompanying fluorescence to produce fluorescent radiation of wavelengths longer than the wavelength of excitation. However, aqueous ink formulations suitable for ink jet printing are not disclosed and markings produced from those compounds do not have adequate stability to sunlight/ultraviolet (UV) light to permit practical marking speeds required for suitable intelligible markings.
The polymeric compositions used in U.S. Pat. No. 5,336,714 are unique in that the near infrared fluorophores (NIRF's) are copolymerized therein and thus are not extractable, exudable, sublimable or leachable from the polymeric composition. The phthalocyanines and naphthalocyanines mentioned therein are unique in that they provide improved UV light stability over the known cyanine laser dyes used in U.S. Pat. No. 5,093,147. However, U.S. Pat. No. 5,336,714 provides no help in formulating an aqueous based ink suitable for ink jet printing using the polymeric compositions containing the copolymerized NIRF compound.
U.S. Pat. No. 4,540,595 provides an ink which fluoresces in the near infrared and which is used to mark documents such as bank checks for automatic identification. The dyes used are phenoxazines (e.g. 3,7-bis(diethylamino) phenoxazonium nitrate is the preferred fluorescent material) which impart blue color to the marked substrate and thus are not invisible.
Certain inorganic rare earth compounds typified by neodymium (Nd), erbium (Er) and ytterbium (Yb) have been used to impart fluorescent markings that can be activated in the infrared to data cards (U.S. Pat. No. 4,202,491). Inks prepared from the insoluble rare earth metals are prone to clogging ink jet nozzles causing poor print start-up and thus, in general, are not practical.
In U.S. Pat. No. 5,093,147 inks are disclosed which are useful for printing infrared fluorescent invisible markings on the surface of an article using certain known polymethine (cyanine) laser dyes. The dyes used, however, have the disadvantage of fading or decomposing upon brief exposure times to ultraviolet light, thus rendering the marking method inferior.
Certain 16,17-dialkoxyviolanthrones (also called dibenzanthrones) are known (U.S. Pat. No. 3,630,941) to be useful as infrared fluorescent markers when solubilized in various substrates, although they are not fluorescent in the solid state. These high molecular weight compounds have essentially no water solubility and thus have no utility for formulation of water based inks for ink jet printing. Furthermore, these compounds have significant absorption of light having wavelengths below 700 nanometers and therefore do not usually provide invisible markings.
Japanese Laid-Open Patent Application: Hei3-79683 discloses ink formulations containing infrared absorbing naphthalocyanine compounds useful for printing barcodes and for identifying documents to prevent falsification and forgery. Various meltable waxes and thermoplastic resins are used as vehicles in combination with alcohols and aromatic hydrocarbons to produce non-aqueous inks. The high molecular weight naphthalocyanine compounds have essentially no water solubility and are not useful for formulating aqueous inks for ink jet printing directly. The marking method mentioned in this application does not utilize fluorescence of the naphthalocyanines when exposed to infrared radiation, but relies merely on absorption of infrared radiation.
U.S. Pat. No. 5,336,714 discloses aqueous coating compositions containing about 20 weight percent to about 35 weight percent of a water-dissipatable sulfopolyester having 0.1 ppm by weight to about 10% by weight of a thermally stable near infrared fluorophoric compound copolymerized therein dispersed in water (65-80 weight percent). The ink formulation disclosed (Example 7) was suitable for coating substrates such as paper with drawdown rods, but is not suitable for ink jet printing because of plugging or clogging of the jets.